The transmission-type liquid crystal display (LCD) exhibits a high contrast ratio and good color saturation. However, its power consumption is high due to the need of a backlight. Under a bright ambient environment, the display is washed out by sunlight. On the other hand, a reflective LCD relies ambient light for displaying information contents. Since it does not require a backlight, its power consumption is reduced significantly. However, its contrast ratio is generally lower and color saturation is much inferior to those of the transmission type. At dark environments, a reflective LCD simply lost its visibility.
A transflective LCD has been taught by Sharp (U.S. Pat. Nos. 6,281,952 B1; 6,295,109 B1; 6,330,047 B1), where each pixel is split into R (reflective) and T (transmissive) sub-pixels. Usually, the R and T area ratio is 4:1, in favor of the reflective display. The transmissive display is intended to only be used for dark ambient conditions in order to conserve power.
Two basic types of transflective LCDs are known: single cell gap (see FIG. 1a hereafter) and double cell gap (see FIG. 1b hereafter).
A recent search in the United States Patent Office directed to the subject matter of the invention hereafter disclosed developed the following eight (8) U.S. patents:
U.S. Pat. No. 6,341,002 to Shimizu et al. describes a double cell-gap approach to obtain equal light efficiency and contrast ratio in Transflective LCDs. The cell gap of the T pixel is twice as large as that of R pixel and requires two phase compensation films;
U.S. Pat. No. 6,108,131 to Hansen et al. describes a technique for converting unpolarized light into a linearly polarized light, which deals with the light source for projection displays, rather than dealing with the display device itself;
U.S. Pat. No. 5,986,730 to Hansen et al describes a display structure that can be used for both reflective and transmissive devices in which a wire grid polarizer is used where the reflective and transmissive displays share the same pixels and covers the whole pixel;
U.S. Pat. No. 4,688,897 to Grinberg et al. similarly describes display devices using a wire grid polarizer as a reflector for reflective display devices;
U.S. Pat. No. 5,764,324 to Lu et al. describes a reflective display device using a conductive transparent electrode (the first substrate) that has the same work function as the reflective electrode (the second substrate) for reducing image sticking;
U.S. Pat. No. 6,281,952 B1 to Okimoto et al., which describes a transflective LCD using two cell gaps for the transmissive and reflective pixels;
U.S. Pat. No. 6,295,109 B1 to Kubo et al describes a LCD with two phase retardation films; and,
U.S. Pat. No. 6,330,047 B1 to Kubo et al describes a LCD device with a LC layer disposed between two substrates with a transmissive electrode in one and a reflective electrode region in the other with a transmissive electrode in correspondence with each of the pixel areas.
For a better understanding of the prior art, reference should be made to FIG. 1a which shows a prior art view of a transflective LCD using a single cell gap, and to FIG. 1b which shows a prior art view of a transflective LCD using a double cell gap. In the single cell gap approach, the same cell gap (d) applies to both R and T modes. The cell gap is optimized for R-mode operation. As a result, the light transmittance for the T-mode is lower than 50% because the light only passes the LC layer once. In the double cell gap approach, the cell gap is d and 2d for the R and T sub-pixels, respectively. In this approach, both R and T can achieve high light efficiency. However, the T mode has four times longer response time than that of the R mode. A common problem for the above-mentioned approaches is that R and T pixels have different color saturation. For R pixels, the ambient light passes the color filter twice, but for T pixels the backlight only passes the color filter once. As a result, their color saturation is different.
It becomes very important to improve the color saturation of devices utilizing transflective LCDs, which is not available from the disclosures of the foregoing citations.